The invention relates to the general field of magnetic disk systems with particular reference to GMR based read heads and the stability of pinned layers therein.
The principle governing the operation of magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as an increase in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said increase being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance.
The magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG. 1. In addition to a seed layer 12 on a substrate 11 and a topmost cap layer 17, the key elements are two magnetic layers 13 and 15, separated by a non-magnetic layer 14. The thickness of layer 14 is chosen so that layers 13 and 15 are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other""s magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material.
If, now, layers 13 and 15 are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction 18 in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from 13 to 15 (or vice versa). However, once these electron xe2x80x98switch sidesxe2x80x99, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one the layers 13 and 15 is permanently fixed, or pinned. In FIG. 1 it is layer 15 that is pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic material, or AFM, (layer 16 in the figure). Layer 13, by contrast, is a xe2x80x9cfree layerxe2x80x9d whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface of a magnetic disk).
The structure shown in FIG. 1 is referred to as a top spin valve because the pinned layer is at the top. It is also possible to form a xe2x80x98bottom spin valvexe2x80x99 structure where the pinned layer is deposited first (immediately after the seed and pinning layers). In that case the cap layer would, of course, be over the free layer.
As discussed above, the pinned layer (typically CoFe or similar ferromagnetic material) in the spin valve structures has to be exchange-biased by an AFM material. When pinned by MnPt or NiMn (AFM materials with high blocking temperature), the pinned layers usually display large anisotropy. The anisotropy field, Hck, is comparable to the pinning field Hpin, both these parameters being distributed over a range of values. These features result in pinned layer loop open and instability. This problem is more severe for the NiCr or NiFeCr seeded SVs in comparison to Ta seeded SVs.
It is also known that SVs made of a synthetic anti-parallel pinned layer (SyAP) can significantly reduce the loop open in the pinned layer. The pinning strength of a SyAP SV is much higher than that of the regular single SV. Typically, the device contains two anti-parallel layers AP1 and AP2 (AP2 being the layer closest to the AFM). These two layers are then coupled together through a layer of Ru and rotate coherently. This causes the Hck effect from AP2 to be greatly reduced. While this approach is a definite improvement on the state of the art, the devices tend to exhibit loop opens (hysteresis) and are susceptible to damage from soft ESD (electrostatic discharges).
It is possible for a device to be subjected to an ESD event during manufacturing. During such an event, the sensor temperature rises and there is also an induced magnetic field acting on the pinned layer, due to the large ESD current which is often as high as 10-50 mA. ESD damage can be categorized as:
a. Excessive temperature rise during the ESD eventxe2x80x94the head resistance increases and is permanently damaged due to inter-diffusion and cannot be recovered. We refer to this as xe2x80x9chardxe2x80x9d ESD
b. The temperature rise is too low for significant inter-diffusion to occur and head resistance does not increase. However, the ESD induced magnetic field may be counter to the pinned layer magnetization and cause pinned layer magnetization rotation, resulting in signal loss, scattering of device properties etc. For most of these cases, it is possible to recover layer rotation. We refer to this as xe2x80x9csoftxe2x80x9d ESD. A key aspect of this problem is that if there is no loop open in the R-H curves, xe2x80x9csoftxe2x80x9d ESD will cause less damage to the head.
A routine search of the prior art was conducted. The following publications of interest were found:
1. S. Mao et al, xe2x80x9cNiMn-pinned spin valves with high pinning field made by ion beam depositionxe2x80x9d Appl. Phys. Lett 69(23)(1996)3593.
2. H. Kishi et al, xe2x80x9cStudy of exchange-coupled bias field in NiFe/PdPtMn thin filmsxe2x80x9d IEEE Trans. Magnetics. V32(5)(1996)3380
3. M. Saito et al, xe2x80x9cPtMn single and dual spin valves with synthetic ferrimagnet pinned layersxe2x80x9d, J. Appl. Phys. V85(8)(1999)4928
4. M. Saito et al, xe2x80x9cPtMn spin valve with synthic ferrimagnet free and pinned layersxe2x80x9d J. Appl. Phys. 87(2000)6974
5. C. Horng et al.xe2x80x9cLow field annealing for the spin valves biased by synthetic antiferromagnetsxe2x80x9d. Application Ser. No. 09/458,727, filed Dec. 13, 1999.
And the following patent references of interest:
U.S. Pat. No. 5,751,521 (Gill) shows a synthetic ferrimagnetic layers (e.g., Ru spacer). U.S. Pat. No. 5,856,897 (Mauri; shows a dual SV sensor with Ru spacers. U.S. Pat. No. 5, 408,377 (Gurney et al. and U.S. Pat. No. 6,134,090 (Mao et al.) show related sensors.
It has been an object of the present invention to provide a spin valve structure that has greater pinned layer stability and reduced pinning reversal relative to similar devices of the prior art.
A further object of the present invention to provide a spin valve structure that exhibits a minimum amount of loop opening in its hysteresis curve.
Another object of the invention has been to provide a spin valve that is highly suitable for use in high density recording.
Still another object of the invention has been to provide a process for the manufacture of said spin valve and pinned layer.
These objects have been achieved by a using a modified pinned layer that consists of two cobalt iron layers separated by a layer of ruthenium, iridium, or rhodium. A key feature of the invention is that this spacer layer is significantly thinner (typically 3-4 Angstroms) than similar layers in prior art structures. Normally, when such thin spacer layers are used, annealing fields in excess of 20,000 Oersted are needed to cause the two cobalt iron layers to become antiparallel. The present invention, however, teaches that much lower annealing fields (spanning a limited range) may be used with equal effect. The result is that a very high internal pinning field is created giving devices of this type greater pinned layer stability and reduced pinning reversal. These devices also exhibits a minimum amount of open looping in their hysteresis curves.